Extractive separation process



United States Patent Office 3,155,598 Eatented Nov. 3, 1964 3,155,593 EXTRAQCTHVE SEPARATIUN PRQKIESS David (lornell, Stillwater, Gilda, and Gail H. Birum, Dayton, @hio, assignors to Monsanto Company, a corporation oi Delaware No Drawing. Filed June 23, 1961, filer. No. 119,049 13 Claims. (Cl. 202-39.5)

The present invention relates generally to the separation, concentration, and/or purification of hydrocarbons having various degrees of unsaturation. It is an object of the invention to separate close-boiling hydrocarbons of the classes of paraffins, monoolefins, diolefins, naphthenes and aromatic hydrocarbons by the use of phosphinylhydrocarbyloxysilicon compounds having the following structural formula:

wherein m is an integer from to 3 inclusive and n equals 4-m, and R represents an valkyl group, preferably containing from 1 to 4 carbon atoms, as the extraction agents. It is also an object of the invention to carry out the said separations by means of an extractive distillation method employing the said phosphorus-silicon compounds as the extracting agent.

in a number of hydrocarbon processing operations including cracking, reforming, aromatizing, and dehydrogenating a wide spectrum of hydrocarbons is formed having various degrees of unsaturation or of solubility parameter, cohesive energy density, or internal pressure. It is therefore desirable to be able to make a type separation in order to remove substantially all of each individual family group of hydrocarbons, i.e., the parafins, monoolefins, diolefins, naphthenes and aromatic hydrocarbons. Further separation such as one olefin from another is also a desired objective. Conventional distillation methods are often poorly adapted to the separation and recovery of such classes of hydrocarbons in view of the small differences in the boiling points of the respective compounds. It has also been found that azeotropic distillation in which the azeotrope agents are added to reduce the boiling point of certain components is impractical because of the separation difficulties between such agents and the compounds with which the azeotrope has been formed.

It has now been found that the use of the abovedescribed phosphorus-silicon compounds makes it possible to conduct extractive separations among the above-described classes of hydrocarbons. In carrying out the present invention the said phosphorus-silicon compounds or a mixture thereof are introduced into a distillation column at a point near the top of the column. In this case the one hydrocarbon fraction is withdrawn as the overhead product, while the other hydrocarbon group is obtained as a bottoms product dissolved in the phosphorussilicon compound as the extractive distillation solvent.

The present method is applicable to the separation of hydrocarbons of the classes of paraffins, monoolefins, diolefins, naphthenes and aromatic hydrocarbons, as well as many individual members within such a class, for example, butene-2 from butene-l. The extractive distillation process using the said phosphorus-silicon compounds yields a vapor fraction containing the more volatile of the said hydrocarbons. The volatility here referred to is that of the hydrocarbon when in solution in the phosphorus-silicon compound, such volatility being the product 7 P where 'y is the activity coeflicient and P is the vapor pressure of the hydrocarbon.

Relative volatility, a, is, therefore, the ratio of the P products for two hydrocarbons.

It has been found that the present method is efiicacious as an extractive process with a wide variety of crude hydrocarbon mixtures. Examples of such starting mixtures include the parafiin, monoolefin, and diolefin mixture obtained in the dehydrogenation of butane and butene in order to produce butadiene as the desired product. Another type of crude hydrocarbon mixture which is readily separated by the method of this invention is the octane-octene mixture resulting from the dehydrogenation of a C fraction. Another type of hydrocarbon fraction which can be separated by the present process is a mixture of 5 carbon atom hydrocarbons obtained in a dehydrogenation of pentenes including isopentane in order to produce isoprene. The impurities in such a crude mixture include normal pentane, pentene-l, 2-methylbutene-l, and 2-methylbutene-2. Another crude hydrocarbon mixture readily separated by means of the present method is the mixture of naphthenes and aromatic hydrocarbons obtained in the aromatizing of normal hexane and the subsequent dehydrogenation of such crude mix ture to produce benzene.

The proportion of the phosphorus-silicon compounds employed in the present extractive separation methods varies over the range of from 0.5 to- 10 moles of the said phosphorus-silicon compounds per mole of the crude hydrocarbon mixture, a preferred range being from 1 to 5 moles. The separation processes may be operated over a Wide range of temperatures such as from F. to 300 F., the upper temperature being limited by the tendency of the hydrocarbon to polymerize rather than by any inherent limitation of the extractive distillation process. The use of vacuum or pressure conditions in addition to atmospheric pressure is also a part of the present invention, such expedients being utilized in accordance with conventional practice in order to aid in the separation of low boiling components or in order to maintain high boiling components in the liquid without undue volatilization.

The apparatus employed in the extractive distillation process is typical of the equipment available in this field. It is obvious that such a distillation process may be conducted with any conventional distillation column of the bubble-plate, packed, or sieve-plate type as may be desired. The selection of the best reflux ratio, size and number of plates and other details of column design necessary in order to obtain the desired degree of parity will be obvious to one skilled in the art having the benefit of the present disclosure. If necessary to prevent or minimize the polymerization of unsaturated compounds, conventional polymerization inhibitors may also be used.

The apparatus employed constitutes a conventional extractive distillation column in which the crude mixture of hydrocarbons is charged to the middle region of a column with reflux being returned near the top of the column, while the overhead vapor fraction is withdrawn as an enriched stream of the materials with the higher degree of saturation (or lower solubility parameter). The phosphorus-silicon compound solvent from any source is introduced into the column at a plate located several plates below the top of the column. The bottoms stream leaving the column contains the material with the lower degree of saturation or mgher solubility parameter, together with the solvent. Where more than one of hydrooarbons is present in the vapor and/ or liquid fractions, these fractions may be separately further treated with the extractive distillation solvent to effect further hydrocarbon separations, or, where the boiling points or miscibilities of the various hydrocarbons are sufficiently different, other techniques such as fractional distillation or solvent extraction separation are suitable. In subsequent extractive distillations the more volatile hydrocarbons(s) is withdrawn as overhead vapors and the less volatile hydrocarbon(s) is withdrawn as liquid bottoms. The mixture of solute and solvent in the bottoms fraction is then separated into its components by conventional stripping or separations means, which may comprise the use of water washing, solvent extraction, distillation, or freezing, by which means one may obtain the bottoms solute in the desired pure state. For example, one may employ a conventional fractionation or stripper column, wherein by simple fractional distillation the solute from the bottoms product is recovered as the overhead fraction of the stripper in pure form. In another type of column the bottoms solute in admixture with the solvent is fed into the middle region of a column, while steam or another heated inert gas is fed to the bottom of the column. The overhead product from such stripping operation is the pure solute, while the solvent is obtained as the bottoms product which is then dried and recycled to the main distillation column, as described above.

It has been found that certain phosphorus-silicon compounds are particularly advantageous in the present process, since these materials are relatively stable against decomposition and are non-reactive with respect to the hydrocarbons as well as any impurities which are conventionally found in such crude mixtures. It is also an advantage that the phosphorus-silicon compounds are generally relatively non-toxic and are relatively inexpensive materials. The use of the phosphorus-silicon compounds as herein disclosed makes it possible to separate close-boiling hydrocarbons in a considerably smaller column than would be required for conventional distillation.

The comparative selectivity of an extractive distillation solvent is best determined by its specific efiiciency with respect to the hydrocarbon pair which are to be separated in the present method. This efficiency may be expressed as the relative volatility of the two hydrocarbons in the presence of the solvent. The equation which expresses this relative volatility (alpha) is:

JML EZQQ EPQL ('YPV) hydrocarbon 2 where (gamma) represents the activity coefiicients defined by the following equation:

In the above equation X and Y represent the mole fractions of a given component in the liquid and vapor phases, respectively, while P and P represent the vapor pressure of the given component, and the total pressure of the system, respectively.

In addition to selectivity, solubility of hydrocarbons in the phosphorus-silicon compounds must be considered. The quantity of hydrocarbon dissolved in the solvent is governed by the temperature and pressure of the separation and by the character of the solvent and the hydrocarbon system.

The term, solubility parameter, is uscd in preference to the term internal pressure or cohesive energy density. These terms are essentially equivalent. Cohesive energy density is the square of the solubility parameter, while internal pressure is 41.311 cohesive energy density.

The definition of the solubility parameter referred to in the previous paragraphs is as follows:

nE internal energy of vaporization, calories/ (g. mole) V molal liquid volume, cc./(g. mole) For the condition of ideal gases, AE may be calculated from handbook values of the latent heat of vaporization, AH The temperatures are expressed as degrees, Kelvin.

AH =latent heat of vaporization, calories/ (g. mole) R:l.987 calories/(g. mole) K.) T=absolute temperature, K.

It has been found by means of solubility measurements, standardized at 77 F., that naphthenes or aromatic hydrocarbons with a solubility parameter greater than about 8.4 are quite soluble in these phosphorus-silicon compounds. Conversely, paraflin or naphthene hydrocarbons with less solubility parameters are much less miscible. It has also been found that the solubility parameters given above are affected by the temperature of the system. Monoolefins and diolefins exhibit much greater miscibility with the phosphorus-silicon compounds solvents than corresponding parafiin hydrocarbons. Accordingly, a separation can be effected through either a difference in degree of saturation or, in the case of parafiin, naphthene and aromatic compounds, through a difference in solubility parameter. In some cases, e.g., a mixture of C hydrocarbons, natural volatility ratios of some constituent pairs are inverted by the presence of the solvent.

The following examples illustrate specific embodiments of the invention:

EXAMPLE 1 A number of hydrocarbon-type mixtures were employed in order to demonstrate the selectivity of [l- (diethoxyphosphinyl)ethoxy]trimethylsilane having the structure 0 ll (CH SiO CLIP (0 CzHs);

as an extractive distillation solvent. These tests were conducted at a number of temperatures as set forth in the table below. At the said temperatures at which the equilibrium measurements were made, at the solvent ratio set forth in the table, the relative volatility of the two components was determined. These values of relative volatility are defined in accordance with the description above.

The table of data also shows the enhancement per plate obtained when using a 10% (mole) solution of the more volatile component of each pair with of the less volatile in an extractive distillation using the above phosphorus-silicon compound as the solvent.

Enhancement per plate is calculated according to the expression aX 1+ (a1)X the percentage enhancement being IOOY. In the expression, Y and X refer to vapor and liquid molar compositions, respectively, of the more volatile hydrocarbon, taken on a solvent-free basis. Thus Y=0.129 as 0pposed to X =0.10O indicates an enhancement of 12.9%.

Table I Solv - Enhance- Pres- Temp., gi Relative ment per Run Solute sure, F. S Volatil- Plate mm. Hg ity (percent) Solutes 1 {n-Ientane 726.3 150 4.7 1.05 10.5 Pentene- 1.00 2 {n-Penr'mc 150 9 l- 07 10. 6 Pentene-l--. 1. n-Pentane... 1. 91 17. 3 2-Methylbutene1 150 9 1.53 14.6 Isoprene 1. O0 4 {n-O 160 9 1.03 9.7 Octane-1 1. 00 5 n-Fievme 150 9 3. 89 30. 2 Cyelohexane 1. 0O Butane 1. 51 14. 4 6 B ene 150 9 1. 63 15. 3 Butadiene 1. 00 Methyloyclopentane 2. 76 23. 5 7 n-Hexane 150 9 6. 55 42.1 Benzene 1. 00 2,4-Dimethylpentane 150 1 8. O8 47. 3 8 Oyclohexane 1. 61 15. 2 Benzene; 1. 00

The method of Example 1 for the determination of relative volatility was also conducted utilizing tris- [l-(diethoxyphosphinyl)ethoxy] methylsilane having the EXAMPLE 2 structural formula:

C Hasi as the extractive distillation solvent.

forth in the table below.

II OCHP(O 0,215),

Selectivities and relative volatilities for the hydrocarbons listed in the table are seen to be substantially less using the acetone/water solvent than the phosphorussilicon solvent.

The phosphinylhydrocarbyloxysilicon compounds used in this invention as extractive distillation agents are prepared by the reaction of an alkyl halosilane, an aldehyde and a trialkylpliosphite, e.g., the product recited in Exampie 1, supra, was prepared as follows:

Acetaldehyde (55 g. or 1.25 moles) was added to a mixture consisting of 108.6 g. (1.0 mole) of trimethylchlorosilane and 166 g. (1.0 mole) of triethyl phosphite a during 0.2 hr. with cooling at -40 C. This reaction mixture was cooled for an hour, then warmed to 80 C.

Distillation gave 246.8 g. (97% theory) of a colorless A demonstration of the effectiveness of the phosphorussilicon compounds described herein as extractive distilla- 0 tion solvents may be made by reference to data on hydrocarbon selectivity and relative volatilities of a 2/1 Vol- T able II Solv . Enhance- Pres- Temp., 63 Relative Inent per Run Solute sure, F. Volatilmm. 11g ity (percent) Salutes n-Pentane 1. 18 11. 6 1 {Pentane-l 725'9 150 1.00 n-Pentane 2. 34 20. 6 2 2-Methy1butene-1 150 9 1. 70 15. 9 2-Methylbutene-2 1.04 10.4

Isrgprene.

11- 826L118- 3 {Cyc1ohexane 150 9 1.00 4 {ili/lHexane n} 150 9 2.87 24.2 ethylcyclopentane 1. 00 Butane 1. 75 16. 3 s Butene 150 9 i 1.82 16.8 Bntadiene 1. 00 Methylcyclopentane. 3. 72 29. 2 r1 n-H 150 9 10. 92 54. 8 Benzene 1. 00 2, 4-Dimethylpentane- 4. 28 32. 2 7 Oyclohexane 150 9 1.32 12.8 Benzene 1. 00

liquid, B.P. 6674 C./0.2-0.3 mm. Rectification in a packed column gave 230.6 g., B.P. 56 C./0.1 to 0.2 111111., 11 1.4204, and analyzed for C H O PSi as follows:

ume ratio acetone/Water solvent as shown in the following table:

Table III I Found l Theory Relative Volatility 42. 42. i

9 2 9. Hydrocarbon 12'09 12 2 Acetone Tris 1-(d1eth- Water fi l g th 11.20 11. 0

p my -e oxy1methy1- silane By varying the proportions of reactants, 1.e., the silane, n-Pentane 1. 4a 2.34 aldehyde and phosphite, and the number and kind of i gggg {38 H8 alkyl radicals on the silane, aldehyde and/ or phosphite,

7 other similar phosphinylhydrocarbyloxy-silicon cornpounds are prepared. Typical compounds of this class include the following:

[1- (diethoxyphosphinyl prop oxy] tripropylsilane [1- (diethoxyphosphinyl ethoxy] triethylsilane 1- (dibutoxyphosphinyl ethoxy] trimethylsilane 1- diethoxyphosphinyl butoxy] trimethylsilane Tris 1- (diethoxyphosphinyl) propoxy] methylsilane Tris[ l-(diprop oxyphosphinyl) ethoxy] ethylsilane Tris 1- (dibutoxyphosphinyl ethoxy] methylsilane Tris 1- (diethoxyphosphinyl butoxy] methylsilane Bis l-(dipropoxyphosphinyl) propoxy] diethyl silane Bis 1- (dibutoxyphosphinyl) ethoxy] diethylsilane Bis 1- diethoxyphosphinyl ethoxy] dimethylsilane Bis[ l-(diethoxy-phosphinyl)butoxy1dimethylsilane While exemplary methods are given for utilizing the phosphorus-silicon compounds disclosed herein as extractive distillation agents, the invention is not to be construed as limited to the specific examples recited.

What is claimed is:

1. The method of separating classes of hydrocarbons from a mixture comprising hydrocarbons of the classes consisting of paratlins, monoolefins, diolefins, naphthenes and aromatic hydrocarbons which comprises contacting the said mixture with a phosphinylhydrocarbyloxy-silicon compound selected from the class consisting of compounds having the following structural formula:

II RmSi QC JHPwR wherein m is an integer from 1 to 3 inclusive and n equals 4-111, and R represents an alkyl radical, in an extractive distillation separation, withdrawing a vapor fraction containing the more volatile of the said classes, and also withdrawing a liquid fraction containing the less volatile classes dissolved in the said phosphorus-silicon compound and thereafter separating the individual hydrocarbons from each of said fractions by conventional means and stripping the said phosphorus-silicon compound from the hydrocarbons dissolved therein.

2. The method of separating paraffins from olefins from a mixture comprising hydrocarbons of the classes consisting of paraffins and olefins which comprises contacting the said mixture with phosphinylhydrocarbyloxysilicon compound selected from the class consisting of compounds having the following structural formula:

II RmSi O CllBiP (O R) wherein m is an integer from 1 to 3 inclusive and n equals 4-m, and R represents an alkyl radical, in an extractive distillation separation, withdrawing a vapor fraction containing the more volatile parafiin and also withdrawing a liquid fraction containing the less volatile olefin dissolved in the said phosphorus-silicon compound and thereafter stripping the said phosphorus-silicon compound from the olefin dissolved therein.

3. The method according to claim 2 wherein the parafiin is pentane and the olefin is pentene.

4. The method according to claim 3 wherein the phosphinylhydrocarbyloxy-silicon compound is [l-(diethoxyphosphinyl) ethoxy] trimethylsilane.

5. The method according to claim 3 wherein the phosphinylhydrocarbyloxy-silicon compound is tris[1-(diethoxyphosphinyl ethoxy] methylsilane.

6. The method of separating diolefins from a mixture comprising hydrocarbons of the classes consisting of parafiins, monoolefins, diolefins, naphthenes and aromatic hydrocarbons which comprises contacting the said mixture with a phosphinylhydrocarbyloxy-silicon solvent selected from the class consisting of compounds having the following structural formula:

wherein m is an integer of from 1 to 3, inclusive, and n equals 4-111, and R represents an alkyl radical, in an extractive distillation separation, withdrawing a vapor fraction containing the more volatile paraflins, naphthenes, monoolefins and diolefins and also withdrawing a liquid fraction containing the less volatile aromatic hydrocarbons dissolved in the said phosphorus-silicon solvent and thereafter contacting the hydrocarbon mixture comprising the vapor fraction with additional amounts of said solvent in a second extractive distillation to remove the more volatile paraffins, naphthenes and monoolefins as a vapor fraction from the less volatile diolefins dissolved in said solvent and stripping the said phosphorus-silicon solvent from the diolefins dissolved therein.

7. The method according to claim 6 wherein the diolefin is isoprene.

8. The method of separating parafiins from naphthenes from a mixture comprising hydrocarbons of the classes consisting of paratrins and naphthenes which comprises contacting the said mixture with a phosphinylhydrocarbyloxy-silicon compound selected from the class consisting of compounds having the following structural formula:

0 ll RmSl 0([3HP (OR):

wherein m is an integer from 1 to 3 inclusive, andn equals 4-m, and R represents an alkyl radical, in an extractive distillation separation, Withdrawing a vapor fraction containing the more volatile parafiin and also withdrawing a liquid fraction containing the less volatile naphthene dissolved in the said phosphorus-silicon compound and thereafter stripping the said phosphorus compound from the naphthenes dissolved therein.

9. The method according to claim 8 wherein the paraffin is hexane and the naphthene is methylcyclopentane.

10. The method of separating an aromatic hydrocarbon from a mixture comprising hydrocarbons of the classes consisting of parafiins, monoolefins, diolefins, naphthenes and aromatic hydrocarbons which comprises contacting the said mixture with a phosphinylhydrocarbyloxy-silicon compound selected from the class consisting of compounds having the following structural formula:

wherein m is an integer from 1 to 3 inclusive, and n equals 4-m, and R represents an alkyl radical in an extractive distillation separation, withdrawing a vapor fraction containing the more volatile hydrocarbons and also withdrawing a liquid fraction containing the less volatile aromatic hydrocarbons dissolved in the said phosphorussilicon compound, and thereafter stripping the said phosphorus-silicon compound from the aromatic hydrocarbons dissolved therein.

11. The method according to claim 10 wherein the aromatic compound is benzene.

12. The method of separating olefinic hydrocarbon isomers from a mixture thereof which comprises contacting the said mixture with a phosphinylhydrocarbyloxysilicon compound selected from the class consisting of compounds having the following structural formula:

0 ll nmsi 0( JHP(0R),

wherein m is an integer from 1 to 3 inclusive, and n equals 1% 4-m, and R represents an alkyl radical in an extractive 13. The method according to claim 12 where the oledistillation separation, withdrawing a vapor fraction confinic hydrocarbon isomers are Z-methylbutene-l in the taining the more volatile of the said olefinic hydrocarbon vapor phase and 2-methylbutene-2 in the liquid phase.

isomers, and also Withdrawing a liquid fraction containing the less volatile olefinic hydrocarbon isomers dissolved 5 References cued 1n the file of thls patent in the said phosphorus-silicon compound and thereafter UNITED STATES PATENTS stripping the said phosphorus-silicon compound from the 2,488,449 rautrn-an Nov. 15, 1949 olefinic hydrocarbon isomer dissolved therein. 2,843,615 Linville July 15, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,155, 598 November 5, 1964 David Cornell et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 20, for "from to 3" read from 1 to 3 column 2, line 52, for "degree of parity" read degree of purity column 3, line 13, for "hydrocarbons(s)" read hydrocarbon(s) columns 5 and 6 Table II second column line 2 thereof, for "Pentane-l" read Pentene-l Signed and sealed this 3rd day of August 1965.

(SEAL) Attest:

ERNEST W. SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. THE METHOD OF SEPARATING CLASSES OF HYDROCARBONS FROM A MIXTURE COMPRISING HYDROCARBONS OF THE CLASSES CONSISTING OF PARAFFINS, MONOOLEFINS, DIOLEFINS, NAPHTHENES AND AROMATIC HYDROCARBONS WHICH COMPRISES CONTACTING THE SAID MIXTURE WITH A PHOSPHINYLHYDROCARBYLOXY-SILICON COMPOUND SELECTED FROM THE CLASS CONSISTING OF COMPOUNDS HAVING THE FOLLOWING STRUCTURAL FORMULA: 